1. Field of the Invention
The invention relates generally to the field of eyepieces for small displays. More particularly, the invention relates to compact imaging systems using folded optical paths to produce a wide field of view of a small display.
2. Description of the Related Art
Liquid Crystal on Silicon (LCoS) micro-displays such as the CMD8X6D and CMD8X6P available from Zight Corporation of Boulder Colo. provide great advantages for compact near-eye applications. LCoS micro-displays produce a high resolution image by changing the polarization state of incident light. In the dark state, a pixel reflects light with substantially no change in polarization. In the bright state, the pixel rotates the polarization state of reflected incident light to the corresponding orthogonal state. By illuminating the display with polarized light and then filtering out nearly all reflected light of that polarization, the display image can be viewed by the human eye. Other miniature displays use either polarization effects or reflectivity changes to produce an image.
Typically, the display is illuminated with pulsed red, green, and blue light while the display is synchronized to the pulsed light source to reflect the appropriate color component of the image. The rapidly alternating red, green, and blue images are blended in human perception to form the full-color image of the display. However, the display can also be illuminated with monochromatic light for data or targeting displays. Such displays are used, for example in helmet, windshield, and visor projection systems as well as in small portable headsets and handsets for private display viewing and for virtual reality systems.
A typical illumination and eyepiece system using pulsed LEDs to illuminate the display and a polarizing beam splitter to conduct the reflected bright light to a viewer is shown, for example, in U.S. Pat. No. 6,038,005 to Handschy et al, FIG. 18A. In that patent, the light from the pulsed LEDs is diffused, then collimated by a Fresnel lens and directed to a polarizing beam splitter cube. The cube reflects polarized light from the LEDs to the micro-display. The polarized light is reflected from the micro-display back toward the beam splitter cube. If the polarization state of the light has been rotated then it will pass through the beam splitter cube to an eyepiece that images the reflected light for the viewer. If the light is reflected from the micro-display without a change in polarization, then it will be reflected by the beam splitter cube away from the viewer and back toward the LED source.
Many applications of LCoS micro-displays require eyepieces that are much more compact and lighter in weight than is possible using the beam splitter cube structure described in the patent mentioned above. At the same time, the eyepiece should provide a wide field of view (preferably greater than 30 degrees diagonal). A large exit pupil is also desired to enable a large population with varying interpupillary distance to view the image without mechanical adjustments. Finally, the eyepiece should meet stringent optical performance criteria, including low distortion, low field curvature, high MTF (modulation transfer function), and small lateral color aberration. An eye relief of at least 25 mm is desired to permit the use of spectacles while viewing the virtual image.
In addition to the matters discussed above, in a binocular system, variations in interpupillary distance should be accommodated to allow for a greater range of viewers. Binocular optical systems can accommodate differences in interpupillary distance (IPD) between people in at least two ways. In one way, small eye-boxes (or exit pupils) can be used, the positions of which are mechanically adjustable to bring the eye-box directly in front of the viewer""s eyes. This is how most field binoculars work. In a second way, large horizontal exit pupils can be created which can cover all normal variations in interpupillary distance between different people without adjustment. Wider eye-boxes are more difficult to design but are mechanically simpler and easier to operate.
A more compact eyepiece suitable for use with reflective displays such as an LCoS micro-display is shown in U.S. Pat. No. 6,046,867 to Rana. This design has a cemented prism block with an internal beam splitter, and an air-spaced Mangin-type mirror. A diffractive surface or element with a positive power is used as an eyepiece component and to reduce chromatic and other aberrations. However, with this positive power surface or element, it is very difficult or cumbersome to provide a long back focal length (BFL). A long back focal length helps to accommodate a frontlight in reflective display systems and allows the designer to provide a short effective focal length for the eyepiece in order to give a wider field of view for the user. In the present invention, a negative power element or surface can be used, as is commonly done in retrofocus lens designs to enhance the field of view.
Secondly, the design in the above-mentioned patent provides a telecentric pupil. A significantly non-telecentric design, for both the frontlight and the eyepiece can significantly improve compactness. Performance can also be greatly enhanced by tailoring the degree of non-telecentricity using a so-called field lens, not shown in the above-mentioned patent, located closest to the frontlight. The field lens can be a separate element, or it can be surface molded into the prism at the surface closest to the display.
A method and apparatus are described that provide an enhanced viewing eyepiece for a micro-display. In one embodiment, the invention is an optical imaging system that includes a prism having a first face directed toward a display to receive light from the display and direct it through a second face, a converging optical element between the second face of the prism and the display, and a reflective converging optical element adjacent the second face of the prism to receive the display light through the second face of the prism and reflect it back into the second face of the prism, the invention further includes a diverging optical element aligned with the reflective converging optical element to receive the display light reflected back into the first prism and direct it to an exit pupil.
Other features of the present invention will be apparent from the accompanying drawings and from the detailed description that follows.